The visual spectrum distribution diagram of pig eyes is shown in Figure 1. The wavelength range of pig visual spectrum is 350-650 nm. There are two response peaks (439nm and 556nm) in the range of 430-450 nm and 540-570 nm. The sensitivity reaches the maximum at 540-570 nm. In addition there is visual response in the ultraviolet band of 350-400 nm. Pigs are not sensitive to red light. The most significant difference between human vision and pig eyes is the ability of the pig eye's retina to perceive ultraviolet light. At the same time pig vision has no visual response to red light with a wavelength greater than 650nm.

Lighting intensity in swine barns is best defined using photon flux density (PFD) as the primary biological metric, with a recommended operational range of 500–1950 nmol/m²·s. For practical on-site management, this range can be approximately converted to 40–150 lux based on human visual perception. A dark period is defined when the ground-level photon flux density falls below 80 nmol/m²·s, corresponding to approximately 5 lux.

The photoperiod plays a critical regulatory role in pig physiology, acting as a biological switch that governs metabolic activity, hormonal secretion, and overall bodily functions. Properly designed light–dark cycles deliver dual benefits: extended light periods primarily stimulate growth-related activities such as feed intake, weight gain, estrus expression, and sow productivity, while continuous dark periods are essential for immune regulation, physiological recovery, and long-term health. The ultimate objective of photoperiod management is to establish a stable biological rhythm, minimize stress responses, and enhance animal welfare.